Identification and cDNA cloning of a Xenopus nucleolar phosphoprotein, xNopp180, that is the homolog of the rat nucleolar protein Nopp140

The monoclonal antibody G1C7, recognises both Xenopus nucleolin and a protein of 180 kDa present in Xenopus oocyte nucleoli. This antibody was used to obtain a cDNA clone encoding the 180 kDa protein now called xNopp180 (Xenopus nucleolar phosphoprotein of 180 kDa). Analysis of the deduced amino acid sequence from this cDNA shows that xNopp180 is almost entirely composed of alternating acidic and basic domains, We show that xNopp180 is heavily phosphorylated and that it contains multiple consensus sites for phosphorylation by casein kinase II and cdc2 kinase. In addition we show that xNopp180 is the 180 kDa antigen recognised by the monoclonal antibody No-114, thus allowing reinterpretation of previous work with this antibody. xNopp180 appears to be the Xenopus homolog of the rat nucleolar protein Nopp140. Nopp140 is a nuclear localisation signal binding protein that shuttles on curvilinear tracks between the nucleolus and the cytoplasm. Possible roles for xNopp180/Nopp140 in ribosome biogenesis are discussed.